1. Field of the Invention
The present invention relates generally to memory devices and, more particularly, to memory devices adapted to receive input data and provide output data synchronized with a common external clock signal.
2. State of the Art
Integrated circuits, including memory and processors, which operate in synchronization with an external clock signal, typically generate an internal clock signal for gating the rippling nature of logic and for staging synchronous steps. Because of the inherent latencies associated with successive levels of propagation, the internal clock signal may be delayed when compared with the external clock signal. Such a delay may cause deterioration in the performance of the device during high-frequency operation. For example, during operation at high frequencies, the access time (i.e., the time required for outputting data after receipt of an external clock signal) may become longer than the time required for generating an internal clock signal from the received external clock signal.
Approaches have been explored for reducing the deterioration of the performance of a memory device at higher frequencies, one of which approach includes synchronizing the internal clock signal with the external clock signal. One synchronization implementation includes a delay locked loop (DLL) which is used as an internal clock signal generator. DLLs use an adjustable delay line comprised of a series of connectable delay elements. Digital information is used to either include or exclude a certain number of delay elements within a delay line. In a conventional DLL, a clock input buffer accepts a clock input signal and transmits the signal to one or more delay lines of delay elements. The delay of the delay path is increased from a minimum setting until the edge of the delayed reference clock is eventually time-shifted just past the next corresponding edge of the reference clock. As an element of a conventional DLL, a digital phase detector controls the delay line propagation delay so that the delayed clock remains synchronized with the external or reference clock.
Conventional DLLs suffer from numerous drawbacks. Loop stability and lock time are very important parameters for DLLs. In order to acquire a quick lock, the phase detector has to update as soon as possible. On the other hand, noise and long loop intrinsic delay require filtering to slow down the update rate due to desirable loop stability. Traditionally, the DLLs preferably operate within a wide frequency range and the loop time delay is dictated by the highest frequency. In short, the loop time delay is translated to be the number of clock cycles the phase detector waits until the next comparison. Under process, voltage, and temperature variations, the response time could be two cycles for low-speed operation and ten or more cycles for high-speed operation.
A need, therefore, exists to improve the performance of DLLs and overcome, or at least reduce, one or more of the problems set forth above.